When navigating through a moving medium such as air or water one must take the motion of the medium into account when setting course for a given location.

An aircraft travels in the direction in which its nose points. If one were to determine the direction to a target and then fly in that direction in still air, one would reach the target. If there were a wind blowing over a large area from a direction other than directly ahead of or behind the aircraft then the aircraft would drift sideways in proportion to the cross component of the wind. If one were to continuously readjust one's heading to point toward the target one would still drift, and thus fly a curving course longer than the line from start to finish, wasting time and fuel.

To arrive at one's intended destination efficiently one would point one's nose at some angle into the wind from the line between the start location and the target such that the vector sum of one's motion and the wind would have a resultant coincident with the line. This angle is the crab angle.

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